Upvote:0
The Mars link at Threedecks.org does not contain an error, but shows Displacement and burthen in Lasts (not tons BM). Lasts are a completely different unit of measure of volume utilized extensively in Scandinavia and the low countries and uses a different calculation to tons.
Upvote:7
The simple answer is that there is no direct or meaningful way of converting burthen tonnage to displacement.
As the article you linked to notes, burthen tonnage was a rough calculation based on the length and beam of a vessel. This calculation varied over time and from country to country. When it was devised, it was intended as a measure of a ship's comparative carrying capacity for the purpose of taxation (at a time when warships were simply merchant ships with more guns). So comparing burthen tonnages would tell you which ship was larger only if you are comparing ships from the same country and era.
The burthen calculation is obviously an approximation and takes no account of the ship's actual hull form or design. It's therefore possible for two ships which have the same length and beam dimensions (but with very different hull forms and loadings) to have the same burthen tonnage but different displacement tonnages. Similarly, two ships of the same displacement (but different hull forms) could have very different burthen tonnage measurements (and length/beam measurements).